This invention relates to an error correction apparatus for use in a digital radio system and a transmitter and a receiver both of which are included in the error correction apparatus.
In general, an error correction apparatus of the type described carries out error correction by using a block code, such as a BCH code or the like, in a digital radio system. As a result, an error which might occur in a transmission path or the like is detected and corrected by a receiver of the error correction apparatus in response to the block code in a manner as mentioned in Japanese Unexamined Patent Publication No. Sho 63-2433, namely, 2433/1988.
More specifically, input data bit signals are successively inputted to a transmitter of the error correction apparatus in the form of continuous data signals which are different from a burst data signal. In the transmitter, a sequence of check bit signals is calculated for a predetermined number of bits, namely, a sequence of the input data bit signals and is multiplexed with the input data bit signal sequence by a multiplexer into a multiplexed signal which has a bit rate different from each of the input data bit signal sequence and the check bit signal sequence. The multiplexed signal is transmitted through the transmission path to the receiver of the error correction apparatus.
It is to be noted here that each bit of the multiplexed signal, namely, the block code, has a period shorter than each bit of the input data bit signals with the check bit signal sequence included in the block code. Such an addition of the check bit signal sequence brings about inevitably widening a frequency bandwidth of the block signal as compared with a frequency bandwidth of the input data bit signals. Such a block code is finally transmitted from the transmitter through the transmission path to the receiver as a radio transmission signal.
With this structure, the radio transmission signal has a wide frequency bandwidth so as to transmit the block code of a wide frequency bandwidth as described above. In other words, the frequency bandwidth of the radio transmission signal is widely spread by adding the check bit signal sequence to the input data bit signal sequence.
On the other hand, a strict frequency bandwidth restriction has been generally entailed on a digital microwave channel which limits redundancy. Such a limitation of redundancy makes it difficult in the digital radio system to use a code which has strong correction ability.
In addition, consideration should be also made about occurrence of serious multipath fading which can not be adaptively equalized. Taking this into account, increasing the bandwidth is undesirable in the radio transmission signal. Therefore, it is preferable in the digital radio system to avoid increasing the bandwidth of the radio transmission signal. Under the circumstances, the radio transmission signal can neither have desired correction ability without an increase of the bandwidth of the radio transmission signal nor can it strengthen correction ability so as to cope with serious multipath fading.